The goal of this proposal is to construct and test a single-cycle SIV (SC-SIV) particle as a source of antigen for generating and maximizing an immune response against SIV infection in the rhesus macaque. The unique characteristic of this novel strategy is that viral replication is only allowed to undergo a single cycle, thereby avoiding the potential risks associated with an inactivated whole virus vaccine or a live attenuated virus vaccine. In addition, this single-cycle nature of replication allows a certain degree of de novo synthesis of viral proteins once the particle enters target cells, thereby allowing stimulation of cytotoxic T lymphocytes (CTLS) in vivo. This type of vaccine candidate is, therefore, particularly appealing because it most closely resembles the live attenuated virus vaccine that provides a high degree of protection against SIV infection in macaques, while eliminating the risks associated with persistent viral replication. By multiple introductions of this single-cycle particle into rhesus macaques, it is hoped that the particles will generate an immune response that would sufficiently mimic in magnitude and spectrum the protective responses observed in macaques infected by the SlVmac239dnef and SlVmac239d3 viruses. Allowing de novo synthesis of viral proteins while limiting viral spread from the infected cell is, therefore, the central component of this new vaccine strategy. Dendritic cells (DCs) are specialized antigen presenting cells and are also potent initiators and stimulators of the immune system. Recent developments have made it possible to isolate large numbers of pure DCs directly from peripheral blood mononuclear cells. Taking advantage of these new developments, the applicants plan to include DCs in their immunization regimen to increase the infectivity and antigenicity of SC-SIV. SIV-specific humoral and CTL responses will be studied to evaluate the effectiveness of this novel vaccine and immunization strategy.